drm/i915: Waterproof verification of gen9 forcewake table ranges

We have to make sure there are no holes in the table in Gen9.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1479388435-12062-1-git-send-email-tvrtko.ursulin@linux.intel.com